This invention relates to an improvement in phase plate optics embracing contact lenses and intraocular lenses. A "phase plate", as employed herein and in the claims, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets in the zones said combination diffracts light to produce a specific wavefront which results in a specific intensity distribution of light at the various order (e.g., 0.sup.th, 1.sup.st, etc.) foci of the zone plate.
This invention concerns contact lenses, and more particularly contact lenses utilizing phase plate optics, such as phase plate bifocals and "tuned"Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. Pat. Nos. 5,210,391; 4,338,005; and 4,340,283. The lens design of Cohen, supra, provides that the radii "r.sub.n " of the annular and concentric zones are substantially proportional to .sqroot.n and that the zones are cut so as to direct light to more than one focal point (herein called a "Cohen lens design").
The Cohen lens design with phase plate optics allows lens constructions which are exceptionally thin. Contact lenses may be designed with phase plate optics in order to achieve a bifocal or multifocal effect. The specific chromatic properties of a phase plate may be incorporated in the design of a contact lens including a contact lens having multifocal properties.
There is also the need in multifocal lenses to have the design capability to vary the intensity of light through the lens to accommodate pupil dilation and constriction. It is known that a pupil can vary from 3 to 6 mm. depending upon the level of ambient illumination. It would be desirable to be able to vary the distribution of energy between distance and near focal point according to the users needs. For example, in dim illumination, the user of a contact lens will be typically engaged in distance viewing such in driving of an automobile. It would be desirable to have a contact lens which accommodates that condition. Conversely, a user may seek to have maximum orientation to then ear focus in the lens yet would require a reasonable intensity of light for distance viewing. It would be desirable to have lenses that can be biased to a users requirements or to light intensity.
The nature of the light intensity problem is illustrated by reference to FIGS. 1 and 2. The lens portion depicted in FIG. 1 is a cross-sectional side view of a portion of a half-wave bifocal phase plate with the echelettes depths h given by the equation: EQU h=w/2(n'-n)
where:
w=wavelength of light
n'=refractive index of the contact lens
n=refractive index of tear layer of eye
In FIG. 1 the individual amplitudes of light a.sub.o and a.sub.1 are formed by the individual echelettes E. The total resultant amplitudes of light A.sub.0 and A.sub.1 formed at the 0.sup.th and 1.sup.st diffractive foci are also shown in FIG. 1. In this illustration, the parallel nature of vectors a.sub.o and a.sub.1 demonstrates that the intensity of light is split equally between the two focal points. The intensities at the 0.sup.th order and the 1.sup.st order diffractive foci are given by: EQU I.sub.o =sinc.sup.2 (1/2) [Intensity at 0.sup.th order diffractive focus] EQU I.sub.1 =sinc.sup.2 (1/2) [Intensity at 1.sup.st order diffractive focus]
It is not necessary for a bifocal phase plate to split the incident light equally between its two diffractive foci when the vectors a.sub.o and a.sub.1 are in parallel. This is shown in FIG. 2 where the cross-section of a portion of a bifocal phase plate shows the echelette depths d which are given by the formula: EQU d=a.h where EQU h=w/2(n'-n) EQU 0&lt;a&lt;2
In the case of the FIG. 2 illustration, the intensity of light is not split equally between the two focal points. The intensities at the 0.sup.th order and 1.sup.st order diffractive foci of this example are derived from the following equations: EQU I.sub.o =sinc.sup.2 (a/2) [Intensity at 0.sup.th order diffractive focus] EQU I.sub.1 =sinc.sup.2 (1-a/2) [Intensity at 1.sup.st order diffractive focus]
In this case, the amplitudes of light A.sub.o and A.sub.1 are shifted in phase to parallel non-vertical aligned amplitudes produced by a half-wave bifocal phase plate. The phase shift e is derived from the equation: EQU e=1-a).pi./2
Though the current developments are significant improvements in the art, there is always a need to improve on the adaptability of the lenses of pupil-diameter variations and decentration. It is desirable to provide bifocal performance of a lens of the Cohen design with the feature that it can shift focussed light from distance to near in coordination with the human eye's pupil, which normally constricts during near viewing.
It has been determined that contact lenses with phase plate optics may generate a few problems for the wearer. One is the glare that results from the non-optical edges of the step between the annularly arranged echelettes that make up a phase plate and appears through wave interference as a disconcerting, intense light to the contact lens user.
Another potential problem stems from (i) the need in soft contact lenses to have sufficient mobility in the lens' fit to the cornea to allow tear fluid exchange to cleanse the surface of the eye of metabolic waste and (ii) the inability of the soft lens to move sufficiently during wearing to satisfy that needed mobility.
The provision of a multiplicity of multifocal Fresnel echelettes in the annular zone plate arrangement of the Cohen lens design in a soft contact lens tends to limit the mobility of the lens. It would be desirable to incorporate into the design of such lenses sufficient mobility that the lens has the capacity of moving about 0.5 to about 1 millimeter of distance during wearing. This would enhance the lens' ability to allow management of the buildup of metabolic waste under the lens.
It is another feature of this invention, amongst other things, to provide a multifocal contact lens design encompassed within the annular arrangement of the Cohen patents, supra, which minimizes the effects of glare from the non-optical edges and/or possesses the requisite mobility during use, as characterized above.